JP3141377B2 - Positive active material for lithium secondary battery and method for producing the same - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an organic electrolyte lithium secondary battery having high energy density using lithium as a negative electrode active material, and more particularly to an improvement of the positive electrode active material thereof.

_2. Description of the Related Art As a lithium battery, a primary battery using MnO ₂ for a positive electrode has already been put to practical use. In the case of a lithium battery, the presence of moisture adversely affects battery performance.
O ₂ was heated at a temperature of 250 ° C. to 400 ° C. to remove adhering water and bound water and used as a positive electrode of a lithium battery.
The crystal structure of MnO _2, was heat-treated at a temperature of as 250 ° C. to 350 ° C. is disclosed in JP-B-49-25571 γ-β
Or β-form which has been heat treated at a temperature of 350 ° C. to 430 ° C. as disclosed in US Pat. No. 4,133,856. However, in subsequent studies, MnO ₂ heat-treated at 400 ° C. in air is also referred to as γ-β type MnO _2, and it is said that bound water cannot be completely removed.

Further, it is said that if the bound water is completely removed, the γ-β type cannot be maintained, resulting in β-type MnO ₂ having extremely low activity as a battery active material. Further, it is known that even when the γ-β type is maintained, the capacitance characteristics deteriorate as the heat treatment temperature increases. This is partly because the surface of the active material partially changed to β type, but it is mainly caused by a decrease in surface activity such as partial reduction of the active material surface. Taking these facts into account, 350
Γ-β type MnO ₂ which slightly leaves bound water heat-treated at a temperature of about 400 ° C. to 400 ° C. is used in a lithium battery.

Problems to be Solved by the Invention However, when MnO ₂ having this crystal structure is used as a lithium secondary battery, the initial capacity is high and the energy density is high, but the capacity decreases with the cycle due to the collapse of the crystal structure due to charge and discharge. Further, it is said that residual bound water flows out due to the collapse of the crystal structure, which adversely affects battery performance, particularly cycle characteristics and storage performance. Further, this active material has a high capacity at normal temperature, for example, in an environment of 20 ° C., but has a very low discharge capacity at a low temperature. For example, at −20 ° C., it has a capacity of about 30% of the capacity at 20 ° C. There is a disadvantage that the capacity is reduced. This is unique to lithium secondary batteries, and for Li / MnO ₂ primary batteries,
There is no significant decrease in capacity at such low temperatures. Therefore, in the current lithium secondary battery using MnO ₂ as an active material, the cycle reversibility, the storage performance, and the low-temperature characteristics are insufficient, and some improvement is considered necessary.

An object of the present invention is to provide a high energy density lithium secondary battery having a stable discharge capacity with respect to the progress of a cycle. Further, the present invention improves MnO ₂ having a high energy density and provides an active material having excellent cycle reversibility, storage characteristics, and low-temperature characteristics.

Means for Solving the Problems The present invention provides Mn: P with an atomic ratio of 1.00: 0.10 to 1.00: 0.02,
An oxide containing phosphorus (P) in manganese dioxide (MnO ₂ ) is used as a positive electrode active material.

Further, MnO ₂ is desirably electrolytic manganese dioxide (EMD). In addition, the manufacturing method is to convert MnO ₂ and P ₂ O ₅ to Mn.
O ₂ : P ₂ O ₅ is mixed at a molar ratio of 1.00: 0.01 to 1.00: 0.05 and calcined in air at a temperature of 400 ° C. or more and 500 ° C. or less. Further, it is preferable to use water as a medium when manganese dioxide and P ₂ O ₅ are mixed, dissolve P ₂ O ₅ in water in advance to make phosphoric acid, and then bake at the above-mentioned predetermined temperature. By using the above-described cathode active material and the production method of the present invention, cycle reversibility, storage characteristics, and low-temperature characteristics are improved while maintaining the high energy density of MnO ₂ , and an excellent lithium secondary battery is obtained. Can be achieved.

The conventional P ₂ O ₅ for work in an attempt to be added to the active material, P ₂ to V ₂ O ₅
There is a method in which O ₅ is mixed and fired to form amorphous V ₂ O _5, and there is a report that this is effective as an active material for a lithium secondary battery. Originally, V ₂ O ₅ is excellent in cycle reversibility, but because it has two stages, one of the stages must be selected in actual use, and the capacity is low regardless of which is selected. Therefore, a high energy density cannot be expected. However, in this technology, V ₂ O ₅ is made amorphous and its discharge form is made into a single-step discharge, and its discharge voltage curve is extremely poor in flatness peculiar to amorphous, but the energy density is improved. That is.

However, the active material of the present invention containing P in MnO ₂ was not amorphous as a result of X-ray diffraction analysis. In addition, the diffraction pattern was confirmed to include a new peak that could not be partially analyzed, but was almost similar to γ-β type MnO ₂ . The discharge form of MnO ₂ was originally a one-step discharge, and the discharge flatness was almost the same as that of the conventional active material of the present invention. Therefore, it is considered that the technique is basically different from the improvement by the amorphization in the case of V ₂ O ₅ . Also, the presence of a new peak suggests that some different crystallization phase has occurred, but details are not clear. However, it is clear that this slight change in the crystal morphology suppresses the collapse of the crystal structure due to the charge and discharge of the γ-β type MnO ₂ , and at least contributes to the cycle reversibility. Also, it is generally said that when MnO ₂ is discharged as a lithium battery, Li electrochemically penetrates into the MnO ₂ crystal and the electron conductivity decreases. further,
In the case of a lithium secondary battery, it is said that all the Li that has entered MnO ₂ by discharging is not desorbed from MnO _{2 by} charging, and part of the Li remains in the crystal. That is,
When used as an active material of a lithium secondary battery, MnO ₂ is used in a state of low electron conductivity. However, when comparing the specific resistance of the active material of the present invention after discharging the conventional γ-β type MnO ₂ with the specific resistance of the active material of the present invention after discharging, it was found that the resistance of the active material of the present invention was low. Was. That is, when P is present as a kind of impurity in MnO ₂ as in the active material of the present invention, a bond is formed between tetravalent Mn and pentavalent P, and the semiconductor is formed by controlling the valence. It can be assumed that: That is, the improvement in the electronic conductivity of the active material itself is considered as one of the causes of the improvement in the low-temperature characteristics. In addition, although it has been described that heat treatment is conventionally performed to remove bound water of MnO ₂ , MnO ₂ is generally treated by heat treatment.
The surface area of O ₂ is significantly reduced (eg EMD in air at 400
Heat treatment at 20 ° C to 20% to 40% of the original surface area).

Further, the surface of the active material is partially β
Or changes to the mold, but the active material surface was already mentioned that the reduction in the surface activity of such partially reduction occurs, lowering of the loss and the surface activity of the surface area affect the low temperature properties of MnO ₂ It is thought that there is. Therefore, for the active material of the present invention, found that the results were compared with the surface area after firing the surface area before firing MnO ₂ (measured by the BET method), it is maintained more than 80% of the original surface area Was. Further, P ₂ O ₅ is expected to act as an oxidizing agent in the firing step, at least
It is considered that the surface of MnO ₂ is hardly reduced and can maintain an active state. That is, this is also considered to be a cause of improvement in low-temperature characteristics. Probably, any of these or these act in combination to contribute to the improvement of low-temperature properties.

Next, regarding the storage performance, in the case of a primary battery, since the performance in a state where no discharge has been experienced is questioned, even in γ-β type MnO ₂ , the bound water is confined in the crystal, and the effect is small. Was. However, when used as a secondary battery, in a battery using the conventional γ-β type MnO ₂ , when the battery is stored for about one month in an environment of, for example, 60 ° C. after several cycles, the internal resistance is remarkably increased. The subsequent charge and discharge was not inconvenient. Then, when the battery after storage was disassembled and observed, it was found that the Li surface of the negative electrode was particularly corroded. This was very similar to what happens when a primary battery is intentionally used with a high water content. That is, γ
In the case of -β-type MnO ₂ , crystal collapse occurs during charge / discharge, and secondary water is bound to flow out. This is thought to be caused by the negative electrode Li, which is particularly vulnerable to moisture, being attacked during storage. However, in the case of the active material of the present invention, storage performance was improved.
One of the reasons for this is considered to be that the active material of the present invention has a slightly different crystal form from that of the conventional active material, and is less likely to collapse due to charge and discharge. Further, in this active material, P is Mn
Although it seems that P is uniformly distributed in O ₂ , it is expected that P exposed on the surface of the active material partially has the form of P ₂ O ₅ . In other words, P ₂ with extremely strong water absorption
Since O ₅ is present on the surface of the active material, the bound water that has flowed out is trapped here, does not move to the negative electrode, and it is conceivable that the negative electrode Li does not corrode.

As described above, some assumptions can be made about the mechanism of various performance improvements. However, by using the active material and the production method of the present invention, cycle reversibility, storage performance, and low-temperature characteristics are simultaneously improved. The facts are very interesting.

Examples Examples of the present invention will be described below.

(Example 1) An oxide composed of Mn and P of the present invention was prepared as follows. First, a predetermined amount of P ₂ O ₅ is dissolved in water, and a predetermined amount of Mn is added thereto.
Add O ₂ powder, mix thoroughly, and partially evaporate water to form a muddy mass.
Baking for 5 hours. Also, any mixing method such as a method in which MnO ₂ and P ₂ O ₅ are mixed in a powder state in advance and then adding and kneading water, and a method in which MnO ₂ powder is added and kneaded in an aqueous phosphoric acid (H ₂ PO ₄ ) solution are used. The same thing was able to be prepared by the subsequent baking. However, if the mixture of powders is directly baked without using water, the reaction is difficult to be performed uniformly, the performance variation becomes large, and the P content after preparation is lower than the amount of P ₂ O ₅ charged. It has happened that the amount has decreased. Conventionally, P ₂ O ₅ has several forms in its crystalline form, one of which is 35
It is said that some substances sublime when the temperature exceeds 0 ° C. Probably, it contained such a form of P ₂ O ₅ . However, it is said that when any type of P ₂ O ₅ is dissolved in water, it becomes orthophosphoric acid, and when it is heated again, it changes into a stable type that is extremely resistant to sublimation.

That is, it can be said that the production method in which water is involved in mixing MnO ₂ and P ₂ O ₅ as in the present invention has an important meaning.

Then, as a result of examining the relationship between the mixing ratio of EMD and P ₂ O ₅ and the ratio of Mn and P in the active material by chemical analysis of the active material,
According to the preparation method of the present invention, within the firing temperature range of the present invention, Mn
It was found that neither P nor P was included in the active material without loss.

(Example 2) Using MMD as MnO ₂ , according to the above-mentioned preparation method, MnO ₂ : P ₂
An active material of the present invention (Mn: P is 1.00: 0.06) mixed at a molar ratio of O ₅ = 1.00: 0.03 and calcined at 400 ° C., and a conventional γ-β type MnO ₂ Material was prepared. First, several button-type batteries as shown in FIG. 2 were assembled for these two active materials, and their characteristics were compared.

In FIG. 2, the positive electrode 1 is composed of an active material in which carbon powder of a conductive agent (5% by weight based on the active material) and polytetrafluoroethylene resin powder of a binder (7% by weight based on the active material) are mixed. This is press-formed on a titanium net 2 fixed by spot welding inside the positive electrode case. The amount of active material was 100 mg in each case. A gasket made of polypropylene together with a separator 5 made of polypropylene, a negative electrode 5 of metallic lithium pressed on a sealing plate 4 and an electrolytic solution 6 (1 mol / LiAsF ₆ dissolved in a mixed solvent of propylene carbonate and ethylene carbonate). 7 sealed through diameter
The battery is 2mm and 1.6mm high. This battery was prototyped for the purpose of comparing the characteristics of the positive electrode. The capacity of the negative electrode was filled about four times the capacity of the positive electrode, so that the charge-discharge characteristics were not affected by lack of the negative electrode. ing. The charge / discharge test was performed at a constant current charge / discharge of 1.0 mA and a charge end voltage of 3.8
V, and the discharge end voltage was set to 2.0 V.

FIG. 1 shows the capacity-cycle characteristics of a battery using the above two types of active materials.

In FIG. 1, curve 8 is the characteristic of the conventional active material, and curve 9 is that of the active material of the present invention. A battery using a conventional active material has a large capacity at the beginning of a cycle, but also has a large capacity decrease accompanying the cycle. On the other hand, those using the active material of the present invention have a small capacity at the beginning of the cycle,
When the cycle is exceeded, the active material of the present invention exceeds the conventional one. Further, as can be seen from the subsequent decrease in capacity, it is apparent that the cycle reversibility of the active material of the present invention is clearly excellent. In particular, the active material of the present invention is characterized by a behavior that the capacity gradually increases at the beginning of the cycle.

Next, with respect to the above two types of batteries, the batteries were taken out in the state of charge at the 30th cycle, stored for one month in an environment of 60 ° C., and changes in internal resistance before and after storage were measured. The internal resistance of all batteries was 5 to 10Ω before storage, but after storage, the internal resistance of the conventional battery was increased to 40 to 50Ω. However, in the battery of the present invention, it only increased to about 10 to 15Ω. Therefore, a charge / discharge test was performed again on the two types of batteries subjected to this storage.

FIG. 3 shows a comparison of the capacity-cycle characteristics when the above-mentioned storage is included in the middle (30th cycle). In the conventional battery (curve 10), the capacity greatly decreases after storage. You can see that. However, in the battery using the active material of the present invention (curve 11), the decrease in capacity was extremely small.
It can be said that the storage characteristics are excellent.

Next, charge and discharge tests were performed on the two types of batteries in a low-temperature environment of room temperature (20 ° C.) and −20 ° C., and their discharge characteristics were compared. FIG. 4 shows the discharge voltage characteristics at the 30th cycle of each battery.
Compared to room temperature (dashed curve 12), the one at -20 ° C (curve)
In 13), the voltage is reduced and the capacity is reduced to 30% or less of room temperature. However, in the battery using the active material of the present invention, the battery at −20 ° C. (curve 15) has a voltage lower than that at room temperature (dashed curve 14), but has a capacity of 70% or more of room temperature. You can see that it is maintained. Therefore, it can be said that the active material of the present invention has excellent low-temperature characteristics.

Next, active materials prepared using chemically synthesized manganese dioxide (CMD) as a raw material together with P ₂ O _{5 under the} same composition and under the same conditions as above were also examined. As a result, the button battery using the same weight of the active material had almost the same discharge characteristics as the case of the EMD, and the capacity did not change. However, as a result of measuring the bulk density, when using a positive electrode having the same shape and the same size as that of the EMD by nearly 20% higher than the EMD (generally, the size is restricted in a practical battery), the advantage of the CMD is small. Therefore, in order to realize a high energy density, the raw material MnO ₂ is preferably EMD.

(Example 3) As described above, since it was found that the active material of the present invention exhibited excellent cycle reversibility, storage performance and low-temperature characteristics, the firing temperature according to the production method was examined next. Mix EMD and P ₂ O _{5 so} that Mn: P becomes 1.00: 0.06,
A battery under the same conditions as in Example 2 was constructed for each active material prepared by changing the firing temperature variously between 300 ° C. and 500 ° C., and a charge / discharge test was performed.

The charge / discharge test was performed at a constant current charge / discharge of 1.0 mA, with the charge end voltage set to 3.8 V and the discharge end voltage set to 2.0 V.

FIG. 5 shows the capacity-cycle characteristics of typical active materials. Firing temperature
Those having a temperature of 300 to 340 ° C have a large initial capacity as shown by the curve 16 (340 ° C) in FIG. 5, but have difficulty in cycle reversibility. Curve 17 (49
(0 ° C.), the cycle reversibility was excellent, but the capacity was small. Curing temperatures of 350 ° C to 480 ° C correspond to curves 18 (350 ° C), 19 (400 ° C), and 20 (45 ° C).
0 ° C.) and curve 21 (480 ° C.), the capacity and cycle reversibility were both excellent. Therefore,
From the capacity-cycle characteristics, it can be said that the firing temperature of the active material of the present invention is preferably 350 ° C to 480 ° C. In addition, all of the active materials of the present invention prepared in this temperature range exhibited excellent storage performance and low-temperature characteristics of the battery as in the above-described examples.

Example 4 Next, the atomic ratio of P of Mn (Mn: P) in the active material was examined. The preparation method of the active material was as described in Example 1, the firing temperature was fixed at 400 ° C., and the Mn: P ratio was 1.00: 0.01 to 1.
Various active materials were prepared between 00: 0.15. Then
For each, a battery under the same conditions as in the above example was constructed and a charge / discharge test was performed.

The charge / discharge test was performed at a constant current charge / discharge of 1.0 mA, with the charge end voltage set to 3.8 V and the discharge end voltage set to 2.0 V. Sixth
The figure shows the capacity of typical active materials.
It shows the cycle characteristics. As is clear from this figure, the characteristics of the active material with Mn: P = 1.00: 0.01 (curve 22)
Was close to the characteristics of the conventional γ-β type MnO ₂ (broken line in the figure), and although the initial capacity was large, the cycle reversibility was difficult. Then
The curve for Mn: P = 1.00: 0.22 to 1.00: 0.10 is indicated by curve 23 (Mn: P =
1.00: 0.02), curve 24 (Mn: P = 1.00: 0.05) and curve 25
(Mn: P = 1.00: 0.10), the initial capacity is small, but the capacity gradually increases with the cycle, and the capacity characteristic of the conventional γ-β type MnO ₂ is reduced in about 10 to 15 cycles. In addition, the cycle reversibility after that was excellent. However, Mn: P = 1.00: 0.11
At に な る 1.00: 0.15, the initial capacity is very small as shown by curve 26 (Mn: P = 1.00: 0.11), and the capacity gradually increases with the cycle, but the conventional γ-β
50 cycles or more must be elapsed before the capacity characteristics of the type MnO ₂ are reached, and there is no advantage in practical use compared with the conventional one. However, from the viewpoint of cycle reversibility, this active material is superior to the conventional one. Therefore, from the capacity-cycle characteristics, it can be said that the range of Mn: P = 1.00: 0.02 to 1.00: 0.10 is preferable. Further, all of the active materials of the present invention having this Mn: P ratio were excellent in storage performance and low-temperature characteristics of the battery as in the above-mentioned examples.

According to the present invention, a lithium secondary battery having high energy density, excellent cycle reversibility, and excellent storage performance and low-temperature characteristics can be provided.

[Brief description of the drawings]

1, 3, 5, and 6 are comparison diagrams of capacity-cycle characteristics, FIG. 2 is a longitudinal sectional view of a battery used in an embodiment of the present invention, and FIG. It is a discharge voltage characteristic diagram. 1 ... Positive electrode, 2 ... Titanium net, 3 ... Separator,
4 ... sealing plate, 5 ... lithium negative electrode, 6 ... electrolyte solution, 7
……gasket.

──────────────────────────────────────────────────の Continuing on the front page (72) Inventor Nobuo Eda 1006 Kadoma Kadoma, Osaka Prefecture Inside Matsushita Electric Industrial Co., Ltd. In-company (72) Inventor Hiromi Okuno 1006 Kadoma Kadoma, Osaka Prefecture Inside Matsushita Electric Industrial Co., Ltd. Document JP-A-2-253560 (JP, A) JP-A-2-155166 (JP, A) JP-A-2-256163 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H01M 4/02-4/04 H01M 4/50-4/58 H01M 10/40

Claims (3)

(57) [Claims] 1. A Mn: P 1.00: 0.02 to 1.00: having an atomic ratio of 0.10, the positive electrode active material for lithium secondary battery by adding phosphorus to MnO _2. (2) MnO ₂ is electrolytic manganese dioxide (EMD), which is mixed at a molar ratio of MnO ₂ : P ₂ O ₅ = 1.00: 0.01 to 1.00: 0.05, and which is mixed at 350 ° C. or more and 480 ° C. or less in air. A method for producing a positive electrode active material for a lithium secondary battery, characterized by firing in a temperature range. 3. The method according to claim 1, wherein water is used as a medium when manganese dioxide and P ₂ O ₅ are mixed, and P ₂ O ₅ is dissolved in water in advance and then fired at the predetermined temperature. 2. The method for producing a positive electrode active material for a lithium secondary battery according to the item 2).